1. Field of the Invention
The present invention is related to dielectric barrier films and, in particular, dielectric barrier films formed from high-density optical material layers for utilization in optical, electrical, tribological, and bio-implantable devices.
2. Discussion of Related Art
Dielectric barrier layers are becoming increasingly important as protective layers for organic light emitting diodes (OLEDs) and other optical or opto-electronic devices. Typically, dielectric barrier layers are deposited thin films with the appropriate electrical, physical, and optical properties to protect and enhance the operation of other devices. Dielectric barrier layers can be utilized in optical, electrical, or tribological devices. For example, touch screen displays require optically transparent protective layers to protect against transmission of atmospheric contaminants as well as to protect against physical wear.
Many thin film deposition technologies that may be utilized to form such dielectric layers include some form of ion densification or substrate bias densification. The densification process eliminates the columnar thin film structure that is typical of vacuum deposited chemical vapor (CVD) or physical vapor deposition (PVD) thin films. It is well known that such densification can be achieved by a secondary ion source arranged to “bombard” the film during deposition. See, e.g., W. Essinger “Ion sources for ion beam assisted thin film deposition,” Rev. Sci. Instruments (63) 11-5217 (1992). See, also, Hrvoje Zorc, et al. Proceedings of the Society of Vacuum Coaters, 41st Annual Technical Conference Proceedings, 243–247, 1998, which discusses the effects of moisture exposure on wavelength shift for electron beam evaporated films (e-beams). In particular, Zorc et al. demonstrated a factor of 15 or so improvement in wavelength shift for electron beam evaporated films (e-beam) as compared to e-beam films deposited with a directed ion beam source after exposure to 30% humidity at 25° C.
D. E. Morton, et al. demonstrated wide-band dielectric pass filters comprised of alternating layers of SiO2 and TiO2 deposited using a “cold cathode ion source” to produce oxygen ions for the purpose of providing “moisture stable stacks of dense optical films of silicon dioxide as the low index material and either titanium dioxide, tantalum pentoxide or niobium pentoxide.” D. E. Morton, et al. Proceedings of the Society of Vacuum Coaters, 41st annual Technical Conference, Apr. 18–23, 1998. The results described by Morton, et al., indicated that room temperature resistance to humidity up to 100% humidity was attained, as measured by the optical performance of single dielectric layers deposited on substrates mounted on a rotating platen. Optical extinction coefficients for the six samples tested in Morton, et al., varied from 0.1 to 1.6 ppt, indicating the presence of significant concentrations of defects or absorption centers in the dielectric layers. Additionally, no film thickness or film thickness uniformity data was reported by Morton, et al., for ion beam energies between 134 and 632 Volts and ion beam current up to 5 amps. Morton, et al., therefore fail to describe a film that would operate as a good barrier layer for optical devices.
Self biased physical vapor deposition, such as ion coating or activated reactive deposition, are well-known means of providing hard wear resistant coatings. However, these coatings are either deposited at several hundred Volts of bias voltage and form penetrating surface treatments with the ion flux penetrating the surface to react with the substrate material, or they are ion assisted for the purpose of decreasing the columnar structure of the film. A “filtered cathodic vacuum arc” (FCVA- reference—http://www.nanofilm-systems.com/eng/fcva_technology.htm) has been used to form a dense film from an ion flux. In this case, ions are created and separated from the neutral vapor flux by a magnetic vector so that only species having a positive charge impinge the substrate. The bias voltage can be preset so that average translational energy ranges from about 50 to several hundred Volts are available. Lower ion energies are not reported due to the problem of extracting and directing a lower energy ion flux with a useful space charge density. Although quite rough due to re-sputtering at the high ion energies, hard protective layers of alumina, and other materials such as tetrahedral carbon, can be deposited with this process on cutting tools and twist drills with commercial levels of utility. Due to the limitation of the coating species to the ion flux, coating rates are low. The best or hardest carbon films are often deposited with the lowest rate of deposition, e.g., 0.3 nanometers per second on substrates up to 12″ in diameter.
Transmission of a ZnO film deposited by FCVA at 600 nm wavelength is increased from about 50% at room temperature to above 80% for single films by increasing the temperature of deposition to above 230° C., with the best transmission at 600 nm of about 90% at a deposition temperature of 430° C. and a substrate bias voltage not greater than about 50 Volts. This high temperature processing indicates the use of a thermal anneal process for repair of ion-induced damage to the films. For FCVA deposition with a 200 Volt bias the transmission is much reduced. FCVA films deposited in this fashion have been shown to be polycrystalline. The defect structures exhibited in the FCVA layer are too large for formation of effective optical barrier layers. Additionally, ion sputtering of crystalline films is dependent on the crystal orientation, leading to higher surface roughness. Defect structures formed in a protective layer can degrade the optical quality of the layer and also provide paths for diffusion of atmospheric contaminations through the layer, compromising the protective properties of the layer.
Ion biased films have shown significant progress toward the goal of providing a satisfactory barrier for protection of electronic and optical films, such as, for example, photovoltaic, semiconducting and electroluminescent films. Particularly organic light emitting diodes, which utilize calcium or other very reactive metal doped electrodes and other hydroscopic or reactive materials, can be protected by such films. However, the most biased process to date, the filtered Cathodic Vacuum Arc Coating Technology or FCVAC process, is reported to produce films with a particle density greater than about 1 defect per square centimeter. It may be that the high resputtering rate at the high voltages used in this process cause surface roughening. Certainly, the presence of a particle represents a defect through which diffusion of water vapor or oxygen can proceed. Also, the roughness of the surface formed by the FCVAC process impacts the stress and morphology and also the transparency and the uniformity of the index of refraction. The resputtered film may flake from the process chamber shields or be drawn to the film surface by the large electrostatic field present in an ion beam process. In any case, the particle defect density for particles greater than the film thickness also determines pin hole density or other defects caused by discontinuous deposition of the film because line of sight films can not coat over a particle that is larger than the thickness of the film, let alone a particle many times greater in size than the thickness of the film.
In the case of ion-bias or self-bias energies exceeding several electron volts, the translational energy of the ion participating in the bias process can exceed the chemical binding energy of the film. The impacting ion, then, can either forward scatter atoms of the existing film or back sputter atoms of the existing film. Likewise, the participating ion can be adsorbed into the growing film or it can also scatter or absorb from the film surface. Sputtering of the existing film and scattering from the existing film are both favored at incoming angles of about 45° from the horizontal. In most ion coating processes, the ion beam is directed at a normal incidence to the surface to be coated. However, as noted, at ion energies exceeding the chemical threshold, and particularly at energies exceeding 20 Volts or so, damage to the film or the substrate resulting from the ion energy in excess of the chemical binding energy is significant, and results in surface roughness, increased optical absorption characteristics, and creation of defects.
In the case of the FCVA process, roughness is an increasing function of the film thickness, increasing from about 0.2 nanometers roughness for a 50 nanometer film to about 3 nanometers for a 400 nanometer Cu film indicating substantial roughening of the polycrystalline copper surface due to differential sputtering by the self biased incoming copper ions. Such a film will scatter light, particularly at the interface between two layers of different refractive index. To date, barrier or dielectric properties of FCVA produced films have not been found.
Charging of the deposited film is also a particular problem with ion beam deposited dielectrics. To date, no low temperature dielectric and also no ion beam dielectric is known that has ever been shown to provide the electrical quality required for a transistor gate layer, for example. The ion beams embed charged ions in the film, leading to large negative flat band voltages and fields that can not be passivated at temperatures below about 450° C. The surface charge of the dielectric layer results in slow accumulation of capacitance, preventing the sharp onset of conduction in a transistor application. Consequently, no as-deposited low temperature dielectric, biased or unbiased, has been proposed for low temperature transistor applications or is known at this time.
Therefore, there is a need for high quality, dense dielectric layers for utilization as barrier layers in optical, electrical, tribological, and biomedical applications.